1. Field of the Invention
The invention relates to a method for detecting corrosion and ferrometals, and, more particularly, to corrosion of ferrometals in boiler system units.
2. Description of the Prior Art
Potential corrosion problems in boiler systems have presented boiler operators with many unanswered questions over the history of operating these boiler systems. Problems have increased as the boiler systems themselves have become more sophisticated. Boiler systems that operate at extremely high pressures and temperatures require pretreatment of waters being used to feed the boilers to eliminate contaminants, such as calcium and magnesium, inorganic salts such as ferrous and ferric salts, silicates and the like.
Corrosion reactions in all boiler systems have been demonstrated by many previous researchers to be affected by many parameters, such as oxygen content, water pH, temperature, pressure, water flow velocity, the formation of certain types of salts and scales, for example hardness scales, iron scales, particularly magnetite, and treatment with boiler water treatment chemicals. The boiler water treatment chemicals are designed to protect the boiler from corrosion, hardness scale deposits, oxygen-induced corrosion, pH excursions, and the like.
Recently, it has been determined that the ferrometals making up boiler system units, which ferrometals can include a number of different types of iron-containing steels, iron alloys, and the like, are faced with particularly difficult corrosion detection problems as far as determining when these boiler system units are experiencing corrosion and determining the reasons why the corrosion is occurring.
The chemistry of iron and its corrosion products in boiler systems has been summarized by W. E. Bornak in an article published in Corrosion (NACE), Volume 44, No. 3, March 1988. Bornak describes the measurement of different types of corrosion of ferrometals in boiler water systems, particularly reviewing investigations of magnetite formation and corrosion related thereto in boilers. Bornak's article primarily summarizes the difficulties caused by corrosion and presents several mechanisms to explain why corrosion is occurring. For example, Bornak speaks of the formation of magnetite through the mechanism of oxygen corrosion and magnetite's protective characteristics when formed on a ferrometal surface in contact with boiler waters, as well as the breakdown of the magnetite layer under certain circumstances.
Bornak broadly describes the difficulties encountered by the production of hydrogen, which is a byproduct of corrosion and which has been traditionally used to measure corrosion rates occurring in an operating boiler. According to Bornak, hydrogen atoms produced through corrosion or through some other side mechanism, can diffuse into the metal and be lost either by direct diffusion or by reactions with various forms of metal oxides encountered by the hydrogen. Hydrogen lost in any of these ways is therefore not available for measurement and the detection of actual corrosion based on hydrogen in an operating boiler is therefore limited.
In the text published by Wiley Interscience Publications, John Wiley and Sons, entitled Corrosion and Corrosion Control, An Introduction To Corrosion Science and Engineering, Third Edition, edited by H. H. Uhlig and R. W. Revie, the basic electrochemical theory of corrosion is described in general for iron and steel corrosion in the presence of aqueous media. In anaerobic solutions, the cathodic reaction is presented as the primary reaction responsible for the formation of hydrogen. Since well-operated boilers are operated in essentially oxygen-free environments, thereby qualifying as anaerobic solutions, the cathodic reaction is the primary reaction that produces hydrogen. However, because of the difficulties mentioned above in using hydrogen as a measurement of corrosion, particularly in an operating boiler in the field, other techniques for accurately measuring real time corrosion phenomena are required.
In an article entitled "Dissolved Hydrogen Analyzer--A Tool for Boiler Corrosion Studies", by Jacklin and Wiltsey, published in Materials Protection and Performance, May 1971, the authors explain their attempts to provide for a dissolved hydrogen analyzer which is essentially the best method of analysis existing today for monitoring corrosion in an operating boiler system. However, because of the difficulties mentioned above, dissolved hydrogen is not a preferred method.
Another article entitled "Hydrogen Analysis as a Method of Corrosion Monitoring in Boilers", by Joneson, published in Combustion, August 1979, points out some of the difficulty in interpreting dissolved hydrogen data and distinguishing from hydrogen formed by generalized and localized corrosion reactions and hydrogen which might be generated from chemical decompositions of boiler water treatment chemicals or from some unknown interference.
In Corrosion, April 1984, Paper No. 57 entitled "Application of Dissolved Hydrogen Measurement Technique for Monitoring of Corrosion at High Pressure Boilers", by Mayer, et. al., the authors state that detection and recognition of conditions leading to accelerated corrosion are essential in reducing short and long term metal loss. The authors emphasize hydrogen measurements should be used frequently, but state that the results of the hydrogen measurements would be no more than a summary indication of many processes which may take place within the system. The authors suggest combining hydrogen tests with other analytical and operational parameters. However, these techniques primarily monitor hydrogen values as well as monitoring operational controls in an operating boiler.
In Corrosion, 1990, Paper No. 184 presented Apr. 23-27, 1990 by Bane, et. al., the authors discuss internal boiler corrosion leading to the formation of magnetite and hydrogen. The paper again emphasizes hydrogen studies that can be useful in determining corrosion in boilers. As a result, even in 1990, hydrogen analysis was being presented as the primary way of determining boiler corrosion. However, as stated above, hydrogen evolution as a means of measuring corrosion suffers from the difficulties cited above, i.e., hydrogen atom reactions with iron oxides, hydrogen gas diffusion into metal substrates and problems of determining the source of the hydrogen measured as being from corrosion, breakdown of chemical additives, or similar sources.
Each of the articles cited above are incorporated herein by reference.
In an attempt to determine what other techniques might be available to measure iron in boiler waters, the following articles and/or patents have come to Applicants' attention.
An article in Analytical Chemistry, Volume 34 (3) March 1962, pages 348-352, teaches the use of 4,7-diphenyl-1,10-phenanthroline for the purpose of determining ferrous ion in the presence of ferric ion. This phenanthroline reagent apparently specifically forms complexes with ferrous ion which complexes have a different absorption spectra than the complexes formed by ferric ion. Although iron corrosion is mentioned in general, there is no mention of boiler waters in the reference.
In Analyst, August, 1975 (Volume 100), pages 549-554, Fadrus, et al, teaches a method for determining ferrous ion in water in the presence of ferric ion using 1,10-phenanthroline. Fadrus teaches that ferric ion complexation does provide interferences. The author masks this interference with what is termed "complexones". His recommended complexone is nitrilotriacetic acid (hereinafter NTA). Again, Fadrus, et. al., are concerned primarily with the analytical chemistry of ferrous and ferric phenanthroline complexes and do not disclose or teach the use of this chemistry in boiler waters.
In U.S. Pat. No. 3,836,331, issued to Stookey, the use of 3-(2-pyridyl)-5-6-bis(4-phenylsulfonic acid)-1,2,4-triazine and certain salts thereof, particularly the sodium salts, as useful reagents for spectrophotometric determinations of iron in water and in other solutions is taught. Stookey, column 1, does speak of the necessity to analyze for iron content in boiler waters because the iron oxide content of the water is an index of the rate of corrosion taking place in the boiler. However, it is then taught that conventional methods for analysis of iron normally involve the collection of iron compounds, solubilizing these compounds by adding, e.g., hydrochloric acid, allowing sufficient time for dissolution, and then adding a reducing agent, such as hydroxylamine or hydroxylamine hydrochloride to reduce the iron to the ferrous form. He then specifies the use of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p'-disulfonic acid, (hereinafter FZ), as a specific complexing agent for ferrous ion. FZ acts as a complexing agent for ferrous ion, ferric ion, cobalt salts, copper salts and the like. It is further taught that FZ acts as a ferrous ion chromogen. Stookey also discloses procedures for monitoring total iron content. The procedures involve collecting samples, dissolving samples, reducing samples specifically by the addition of reducing agents, such as hydroxyl ammonium chloride, adding reagents suitably buffered and measuring iron content by colorimetric means.
U.S. Pat. No. 3,770,735, issued to Stookey, describes FZ compounds but primarily emphasizes the synthesis of these compounds. No mention of actual use in boiler waters is made except by hypothesis that boiler water iron content may be an example where such an analysis might be necessary.
In Talanta, Volume 31, No. 10A, pages 844-845, 1984, Li Shi-Yu, et. al, describe the use of ion exchange colorimetry to determine microamounts of iron in water with FZ.
In Analytical Chemistry, Volume 42 (7) June 1970, the advantages of using FZ in combination with a digestion procedure for the determination of total iron in various waters is taught. No mention of boilers is made.
Finally, an article appearing in Analytical Chemistry, Volume 48(8) July 1976, pages 1197-1201, provides for the characterization and application of FZ as a ferrous ion indicator. A number of references are given showing the use of an FZ reagent and the combination of the reagent in a number of automated analyzers and in a test kit for determination of ferrous ion, primarily in blood serum. In a list of cited references the use of FZ in potable water, sea water, plant solutions, plant materials, high purity reagent chemicals, bathocuproine sulfonate analysis, as well as to determine cobalt, ruthenium, osmium and other metal contents are listed. These various analytical uses take advantage of the fact that the ferrous ion/FZ complex is colored and the ferric ion/FZ complex is not colored. Using this characteristic, FZ has also been used to indirectly detect the presence of ascorbic acid in fruit juices, blood serum and urine, to detect the presence of sulfur dioxide in various liquid samples and after absorption from various gases and various enzymatic activities in the NADH/NAD redox systems. No use in boilers is summarized therein.